This invention relates to a thin-film transistor which is used for an active display device using a liquid crystal, for instance.
Now, a prior art active liquid-crystal display device using thin-film transistors will be described with reference to FIG. 1.
The device comprises opposed transparent substrates 11 and 12 of glass or the like which are spaced apart by a spacer 13 provided along their edges and constitute a liquid-crystal cell 10 with a liquid crystal 14 sealed between them. A plurality of display electrodes 15 are formed on the inner surface of the substrate 11. Also, thin film transistors 16 are each formed as a switching element adjacent to each display electrode 15 with the drain connected thereto. A transparent common electrode 17 is formed on the inner surface of the other substrate 12 such that it opposes the display electrodes 15.
Each of the display electrodes 15 constitutes a picture element, that is, a pixel, for instance. As shown in FIG. 2, the display electrodes 15 are square in shape and are arrayed in a matrix on the transparent substrate 11. Gate bus lines 18 are each formed in the vicinity of and along each row of display electrodes 15 in the matrix. Source bus lines 19 are each formed in the vicinity of and along each column of display electrodes 15 in the matrix. Thin-film transistors 16 are formed at the intersections of the gate and source bus lines 18 and 19. Each thin-film transistor 16 has its gate connected to the associated gate bus line 18, its source connected to the associated source bus line 19, and its drain connected to the corresponding display electrode 15.
When a voltage is applied between a selected one of the gate bus lines 18 and a selected one of the source bus lines 19, the corresponding thin-film transistor 16 is turned on, whereby the corresponding display electrode 15 is charged through the thin film-transistor 16. As a result a voltage is applied across only that portion of the liquid crystal 14 that is found between the corresponding display electrode 15 noted above and common electrode 17, whereby the area of that display electrode 15 is rendered transparent or light-blocking. In this way, a selective display of display electrodes 15 is obtained. The display can be erased by causing discharge of the display electrode 15.
In the prior art, the thin film transistor 16 is constructed as shown in FIGS. 3 and 4. As is shown, display electrodes 15 and source bus lines 19 of a transparent conductive film of ITO or the like are formed on the transparent substrate 11. An amorphous-silicon (a-silicon) or like semiconductor layer 21 is formed such that it strides the gap between opposite edges of the display electrode 15 and an associated source bus line 19. A gate insulating film 22 of silicon nitride or the like is formed on the semiconductor layer 21. A gate electrode 23 is formed on the gate insulating film 22 above the semiconductor layer 21 such that the gate electrode 23 partly overlies the display electrode 15 and source bus line 19. The gate electrode 23 has one end connected to the gate bus line 18. Portions of the display electrode 15 and source bus line 19 facing the gate electrode 23 constitute drain and source electrodes 15a and 19a, respectively. The thin-film transistor 16 is constituted by the drain and source electrodes 15a and 19a, semiconductor layer 21, gate insulating film 22 and gate electrode 23. The gate electrodes 23 and gate bus lines 18 are formed simultaneously from aluminum, for instance.
The semiconductor layer 21 has a photoconductive effect, that is, it becomes conductive when it is exposed to light. Therefore, when external light is incident on the semiconductor layer 21 while the thin-film transistor 16 is "off", the "off" resistance thereof decreases, and therefore the "on"-to-"off" current ratio thereof is reduced. Accordingly, a light-blocking layer 25 is formed on the substrate 11 such that it faces the semiconductor layer 21, as shown in FIG. 4. A protective layer 26 of an insulating material, e.g., silicon dioxide, is formed on the protective layer 26. Drain and source electrodes 15a and 19a are formed on the protective layer 26. Further, ohmic contact layers 27 and 28 of n.sup.+ -type amorphous silicon or the like are formed to provide for satisfactory ohmic contact of the drain and source electrodes 15a and 19a with respect to the semiconductor layer 21. The semiconductor layer 21 is formed on this structure. Further, a protective layer 29 of silicon nitride or the like is formed to cover the thin-film transistors 16, source buses 19 and gate buses 18 to protect the liquid crystal.
In the prior art thin film transistor shown in FIG. 4, a transparent conductive film eventually constituting the drain and source electrodes 15a and 19a is formed on the transparent substrate 11, an n.sup.+ -type amorphous silicon layer eventually constituting the ohmic contact layers 27 and 28 is formed on the transparent conductive film, and then the n.sup.+ -type amorphous silicon layer and the transparent conductive film are etched to predetermined patterns, thus forming the display electrodes 15 and source buses 19 and the ohmic contact layers 27 and 28 over the drain and source electrodes 15a and 19a, as shown in FIG. 4. For this reason, when the widths w1 and w2 of the contact portions of the electrodes 15a and 19a in contact with the semiconductor layer 21 are reduced, the ohmic contact of these electrodes 15a and 19b with respect to the semiconductor layer 21 becomes insufficient, and the serial resistance Rs between the electrodes 15a and 19a is increased. Further, since the ohmic contact layers 27 and 28 are formed on purpose, the resistance Rs between the source and drain is increased to an extent corresponding to the thickness of the ohmic contact layers.
Further, in the prior art the transparent drain and source electrodes 15a and 19a consist of tin oxide or ITO (indium oxide and tin oxide), and the ohmic contact layers 27 and 28 and semiconductor layer 21 are formed by a plasma CVD (chemical vapor deposition) process, for instance, on the transparent electrodes. The element constituting the transparent electrodes 15a and 19a, e.g., indium or tin, is diffused as impurity into the semiconductor layer 21 and ohmic contact layers 27 and 28, so that the semiconductor layer 21 is liable to become p-type. Also, the oxygen in the transparent electrodes 15a and 19a is liable to enter the semiconductor layer 21 and ohmic layers 27 and 28 to form silicon oxide. Further, if indium or tin enters the ohmic contact layers 27 and 28, it means that a p-type impurity is introduced into the n.sup.+ -type layer. In such cases, the effect of the ohmic contact is deteriorated, resulting in an increase of the resistance Rs noted above. For the above reasons, thin-film transistors having satisfactory characteristics could not have heretofore been obtained.
If a liquid-display device of the type described above is to have a large display area and a high resolution of display, a large number of display electrodes 15 should be formed at a high density. Also, the source bus lines 19 should be of a considerable length. This means that because of a voltage drop the potential at a point on each source bus line 19 becomes lower as the point departs from one end of the source bus line connected to the voltage supply. In other words, a brightness gradient is produced on the display such that the brightness of a pixel becomes lower as the pixel is located farther away from the end connected to the voltage supply terminal. From these points, it is desired that the source bus 19 has a sufficient thickness. More specifically, it is desired to provide a sufficient thickness of the source bus 19 to reduce the resistance thereof so that the same brightness can be obtained over the entire display surface.
Further, in order to avoid influence of external light on the thin-film transistor due to the photo-conductive effect of the semiconductor layer 21, the semiconductor layer 21 is desirably as thin as possible. In the prior art, a transparent conductive film eventually constituting the electrodes 15a and 19a is formed, an n.sup.+ -type layer eventually constituting the ohmic contact layers 27 and 28 is formed, and then these layers are etched to a predetermined pattern to form the display electrodes 15 and source buses 19. To obtain a high density pattern at this time, the layers are etched by an anisotropic etching process, in which etching mainly proceeds only in the direction perpendicular to the transparent substrate 11. This means that the etched side surfaces of the source buses 19 are almost perpendicular to the substrate 11. Therefore, if the source bus 19 is formed such that it is sufficiently thick, the deposition of the semiconductor layer 21 on the side surfaces of the bus 19 is liable to be insufficient. For this reason, the thickness of the semiconductor layer 21 has to be above a certain value; heretofore, it should be about 1,000 angstroms, for instance. Because of the comparatively large thickness of the semiconductor layer 21 as noted above, the thin-film transistor has been greatly influenced by the photoconductive effect of the semiconductor layer 21. This means that the light-blocking layer 25 as noted before in connection with FIG. 4 has been necessary. The manufacturing process is thus complicated by that requirement. Further, with pattern density increase the width of the ohmic contact layers 27 and 28 is reduced. This leads to unsatisfactory ohmic contact between the electrodes 15a and 19a and semiconductor layer 21. More specifically, this leads to an off-set in the drain current versus drain voltage characteristic of the thin-film transistor 16, that is, the drain current is not caused unless the drain voltage exceeds a certain level. Therefore, the gradation control range of the liquid crystal display device for gradation display is reduced.